Blender 2.8 - How to bake an ambient occlusion map to a second UV texture on a multi material mesh?
Add a new Image Texture node (with a new image for bake) inside each material and keep it selected (active).



To bake into a new UVMap, create one under Data Properties > UV Maps and enable Camera icon.



Then in Render Properties > Bake, change Combine pass to Ambient Oclusion. After image texture is baked you can use it as usual.